In a heat pump system running in a heating mode, it is common for frost to form on an exterior coil of the heat pump system. While the heat pump system is operating in the heating mode, the exterior coil can become extremely cool as the heat pump system attempts to transfer heat from exterior ambient air to a refrigerant in the exterior coil. If a temperature of the exterior coil cools to a temperature below a dew point temperature of the exterior ambient air, condensation occurs on the exterior coil. If the temperature of the exterior coil drops to a temperature below freezing or the exterior ambient air is below freezing, the condensation will turn into frost on the exterior coil. Formation of frost on the exterior coil is common in most areas where heat pump systems are used.
The formation of frost on the exterior coil reduces the effectiveness of the exterior coil as a heat transfer unit. The exterior coil is designed to transfer heat from the exterior ambient air to the refrigerant inside the exterior coil. To achieve this function, an exterior fan is typically used to draw exterior ambient air across the exterior coil. When frost forms on the exterior coil, an ability of the exterior fan to draw air across the exterior coil is reduced, which reduces the exterior coil's ability to absorb heat from the exterior ambient air.
Methods have been developed to defrost the exterior coil to remove frost that has built up on the exterior coil. One defrost method involves switching the heat pump system into a defrost mode during which the heat pump system operates as an air conditioner to transfer heat from the interior of an enclosed space, such as, for example, a house, to the exterior coil to melt any frost that has formed thereon. The heat pump system then operates as a typical air conditioner to transfer heat from the interior of the house to the exterior coil via a compressor and expansion valve system. In the defrost mode, the refrigerant in the exterior coil becomes warmer such that frost that has formed on the exterior coil melts. Meanwhile, the refrigerant in the interior coil becomes cooler. Interior air that is passed over the cooled interior coil blows out into the heated space. This is known in the industry as “cold blow.” Cold blow is typically counteracted with auxiliary heating elements.
When the heat pump system initiates a defrost cycle to remove frost from the exterior coil, three events typically occur: 1) the exterior fan is deactivated; 2) a reversing valve shifts from the heating mode to the defrost mode; and 3) the auxiliary heating elements are activated. The exterior fan is deactivated to stop the cooling effect on the frost formed on the exterior coil and to allow the frost to melt. The reversing valve is shifted to reverse the flow of the refrigerant within the heat pump system to provide hot refrigerant to the exterior coil to melt the frost. The auxiliary heating elements are activated to heat the interior air that is blown over the cool interior coil and into the interior of the building in order to provide warm air.